The invention relates to a level sensor for a container, preferably for a washing water container in a motor vehicle, having electrodes for measuring electrical variables of a liquid in the container and having an evaluation unit for evaluating the measured electrical variables, a pair of reference electrodes provided to register a reference variable being provided for arrangement in the lower region of the container, and a pair of measuring electrodes provided to register measured variables dependent on the level extending over at least a subarea of the height of the container. Furthermore, the invention relates to a method of registering a level in a container, preferably in a washing water container of a motor vehicle, in which a pair of reference electrodes measures an electrical reference variable in the lower region of the container, a pair of measuring electrodes detects an electrical measured variable over the height of the container to be detected, and an evaluation unit calculates the level in the container from the measured electrical variables.
Level sensors of the type mentioned at the beginning are often arranged in current washing water containers and are known from practice. The level in the container results from the comparison of the measured variable determined on the measuring electrodes with the reference variable determined by the reference electrodes. Since the reference electrodes must continuously be located within the liquid in the container, these electrodes are arranged exclusively in the lower region of the container. Therefore, as compared with the pair or measuring electrodes, the pair of reference electrodes has particularly small dimensions. The disadvantage here, however, is that inhomogeneities of the liquid in the container have a very severe influence on the determined reference variable. For this reason, a comparison between the reference variable and the measured variable is less meaningful for the level determined. In particular in washing water containers for motor vehicles, there are often very extensive inhomogeneities, since the washing liquid is a mixture of antifreeze, detergent and water which changes frequently and is difficult to homogenize.
Thought has already been given to extending the pair of reference electrodes over the entire lower half of the container and to extending the pair of measuring electrodes over the upper half of the container. However, the disadvantage here is that, when the container is less than half full, no measured variable can be determined.
The invention is based on the problem of configuring a level sensor of the type mentioned at the beginning in such a way that it measures the level in the container as accurately as possible, even in the case of inhomogeneous liquids. Furthermore, the most accurate possible method of registering the level in a container is to be determined.
According to the invention, the first-named problem is solved in that, in relation to a section of the height of the container, the pair or reference electrodes has a greater surface than the pair of measuring electrodes.
As a result of this configuration, the reference variable is averaged over a particularly large region in an envisaged height of the container. Therefore, physically closely limited inhomogeneities of the liquids cannot distort the reference variable. The level in the container determined by means of the comparison between the measured variable and the reference variable is therefore particularly accurate. The electrical variables determined can be, for example, the electrical resistance, a capacitance or comparable variables between the electrodes. A further advantage of the invention is that the reference electrodes can be arranged close to the bottom region of the container. This permits the registration of measured variables by the pair of measuring electrodes over a particularly high height range of the container.
According to an advantageous development of the invention, the reference electrodes can have a particularly large surface in a limited section of the height if the pair of reference electrodes has a horizontal section.
In the event of slopping of the liquid in the container, according to another advantageous development of the invention, even inhomogeneities present in layer form are averaged in the reference variable if the pair of reference electrodes extends over the entire width of the container.
The level sensor according to the invention requires a particularly low expenditure on construction if at least one of the reference electrodes is configured as an electrical conductor insulated exclusively in the upper region of the container.
A minimum distance between the horizontal section of the reference electrodes and the bottom of the container may be ensured in a simple way, according to another advantageous development of the invention, if the pair of reference electrodes has at its free end an angled portion pointing in the direction of the container bottom.
It is often the case that the quantity of liquid in the container is displayed in the case of a full to virtually empty container. In the case of a virtually empty container, on the other hand, a warning signal is generated which, for example, is intended to warn the driver of the motor vehicle about a threatened failure of the washing system. In this case, there is always still a reserve quantity of approximately one to one and one half liters present in the washing water container. According to another advantageous development of the invention, the reserve quantity in the container can be determined particularly accurately if the pair of reference electrodes is arranged completely within a reserve region of the container.
The pair of measuring electrodes could, for example, be led as far as the bottom region of the container. However, according to another advantageous development of the invention, the reserve region may be determined particularly accurately if at least one of the electrodes of the pair of measuring electrodes is led as far as immediately before the reserve region. In this way, in the case of a measurement of the electrical resistance by means of the pair of measuring electrodes, the start of the reserve region is defined by an abrupt rise in the electrical resistance.
According to another advantageous development of the invention, it contributes to the further enlargement of the surface of the reference electrode if the pair of reference electrodes is of lamellar configuration.
The second-named problem, namely the provision of the most accurate method possible of registering the level in a container, is achieved, according to the invention, in that the registration of the electrical reference variable is carried out over a larger horizontal region of the container than the registration of the electrical measured variable.
In the case of a virtually empty container, it may be necessary to determine the residual quantity that is still present, in order for example to switch off the pump in order to protect it against damage, before the container has been emptied completely. According to another advantageous development of the invention, the determination of the residual quantity in the container requires a particularly low outlay on construction if, in the reserve region of the container, the level is calculated via the duration of the pump operation and the volume flow delivered. The volume flow may be determined, for example, by trials.
According to another advantageous development of the invention, a complicated determination of the duration of the pump operation may be avoided if, in the reserve region of the container, the level is determined via the change in the electrical reference variable. In this case, a proportional change in the level may approximately be assumed as a function of the change in the measured reference variable.